O 
Q 


EXCHANGE 


Colloidal  Solutions  of  Copper 
Sulphide 


A  THESIS 


PRESENTED  TO  THE  FACULTY  OF  CHEMISTRY  OF  STANFORD 

UNIVERSITY  IN  PARTIAL  FULFILLMENT  OF  THE 

REQUIREMENTS  FOR  THE  DEGREE  OF 

DOCTOR  OF  PHILOSOPHY 


By 
ROLAND  NEAL 


Colloidal  Solutions  of  Copper 
Sulphide 


A  THESIS 


PRESENTED  TO  THE  FACULTY  OF  CHEMISTRY  OF  STANFORD 

UNIVERSITY   IN  PARTIAL  FULFILLMENT  OF  THE 

REQUIREMENTS  FOR  THE  DEGREE  OF 

DOCTOR  OF  PHILOSOPHY 


By 
ROLAND  NEAL 


COLLOIDAL  SOLUTIONS  OF  COPPER  SULPHIDE 


BY   ROLAND   NEAI, 

With  the  discovery  that  certain  natural  copper  sulphides 
yield  colloidal  copper  sulphide  on  treatment  with  hydrogen 
sulphide,1  and  the  probability  that  colloidal  solutions  of 
copper  sulphide  may  play  a  not  unimportant  part  in  the  pro- 
cesses involved  in  the  secondary  enrichment  of  copper  sul- 
phide ore  beds,  a  knowledge  of  the  properties  of  such  solu- 
tions from  the  chemical  point  of  view  becomes  a  matter  of 
considerable  interest.  In  the  following  paper  are  given  the 
results  of  a  preliminary  investigation  of  such  solutions,  and 
while  the  researches  are  by  no  means  to  be  considered  as  com- 
plete, considerable  material  of  interest  is  already  in  hand  and 
given  here. 

The  composition  of  the  sulphide  content  of  a  colloidal 
solution  of  copper  sulphide  is  at  the  present  time  a  very  un- 
certain matter.  Older  investigators  maintain  that  by  the 
action  of  hydrogen  sulphide  on  cupric  compounds  in  the 
presence  of  water,  a  mixture  of  cupric  and  cuprous  sulphides 
is  formed,  while  Posnjak,  Allen  and  Merwin2  claim  that  only 
cupric  sulphide  is  thus  formed.  On  the  other  hand,  it  has 
been  observed  in  this  laboratory  that  sulphide  precipitated 
from  cupric  sulphate  by  hydrogen  sulphide  with  exclusion  of 
air  and  sealed  up  in  an  atmosphere  of  hydrogen  sulphide 
shows  the  presence  of  distinct  crystals  of  sulphur  after  some 
months'  standing.  There  seems  to  be  at  least  some  uncertainty 
as  to  the  nature  of  the  sulphide  in  the  solutions  used  in  this 
work,  all  of  which  were  prepared  from  cupric  compounds. 

The  scope  of  the  present  investigations  cover:  (a)  the 
preparation  of  colloidal  copper  sulphide  solutions  (copper 

1  Clark:  Bull.   University  of  New  Mexico,    1914,   Chem.   Series,   Vol.    i, 
No.  2. 

2  Jour.  Econ.  Geol.  10,  528  (1915). 


365231 


4  Roland  Neal 

sulphide  sols);  (b)  influence  of  electrolytes  on  the  dispersion 
of  amorphous  copper  sulphide;  (c)  flocculation  by  electrolytes; 
(d)  influence  of  hydrogen  sulphide  on  the  flocculation  by 
electrolytes;  (e)  migration  velocity  in  the  electric  field;  (/) 
influence  of  electrolytes  and  of  hydrogen  sulphide  on  the 
migration  velocity. 

Experimental 

Methods  of  Preparation  of  Copper  Sulphide  Sols. — For  the 
most  part  the  methods  of  preparation  used  in  this  work  are 
those  which  have  already  been  used,  and  are  in  the  main  de- 
scribed in  Svedberg's  "Herstellung  kolloider  Losungen 
anorganischer  Stoffe."  Solutions  were  prepared  in  a  variety 
of  ways  in  order  that  it  might  be  determined  what  influence 
the  mode  of  preparation  had  on  the  properties  of  the  product. 
Colloidal  solutions  were  therefore  prepared  by  the  following 
three  methods : 

1.  Agitation    of     well-washed     copper     hydroxide    with 
hydrogen  sulphide  water. 

2.  Agitation    of     well-washed     copper     carbonate    with 
hydrogen  sulphide  water. 

3.  Agitation  of    well- washed  freshly  precipitated  copper 
sulphide  with  hydrogen  sulphide. 

In  all  cases  the  solution  was  freshly  saturated  with  hydro- 
gen sulphide  from  time  to  time  and  the  containers  were  kept 
carefully  stoppered  to  exclude  air.  They  were  also  kept  out 
of  direct  light  which  accelerates  the  oxidizing  action  of  any 
air  that  may  gain  admission.  The  following  sols  were  pre- 
pared as  specified  and  in  the  latter  part  of  this  work  will  be 
referred  to  by  the  numbers  given  them  here. 

Sol  i. — Two  grams  of  copper  sulphate  were  dissolved 
in  300  cc  of  distilled  water,  and  the  solution  brought  to  faint 
permanent  alkalinity  with  sodium  hydroxide.  The  pre- 
cipitated hydroxide  was  filtered  off  and  thoroughly  washed 
with  distilled  water.  It  was  then  placed  in  a  large  volume  of 
freshly  distilled  water  and  allowed  to  stand  for  ten  months, 
after  which  the  water  was  decanted  off  and  800  cc  of  fresh 
water  added.  On  now  passing  hydrogen  sulphide  into  the 


Colloidal  Solutions  of  Copper  Sulphide  5 

liquid,  the  conversion  of  the  hydroxide  into  finely  dispersed 
sulphide  began  almost  immediately.  For  a  period  of  ten  days, 
hydrogen  sulphide  was  passed  through  the  solution  at  frequent 
intervals,  and  the  solution  was  subjected  to  gentle  agitation 
for  a  period  of  an  hour  after  each  charging  with  hydrogen 
sulphide.  This  sol  proved  to  be  perfectly  stable.  After 
standing  for  two  and  one-half  months  there  was  no  evidence 
of  any  tendency  to  settle.  The  color  was  very  dark  even 
when  the  solution  was  diluted  i  to  5  with  water. 

Sol  2. — This  was  prepared  similarly  to  the  above,  except 
that  the  copper  was  precipitated  as  carbonate  (by  sodium 
carbonate)  instead  of  as  hydroxide.  The  carbonate  was, 
however,  washed  less  thoroughly  than  the  hydroxide,  the 
whole  preparation  in  this  case  being  completed  within  two 
weeks.  In  general  appearance  the  sol  was  similar  to  Sol  i. 
Kept  saturated  with  hydrogen  sulphide  and  stoppered,  it 
showed  no  deterioration  and  no  settlement  after  many  months. 

Another  preparation,  identical  with  Sol  2,  but  in  which 
the  copper  carbonate  was  washed  by  repeated  boiling  with 
water,  was  not  so  successful,  the  dispersion  with  hydrogen 
sulphide  being  both  less  rapid  and  less  complete.  This  sol 
was  not  used  in  any  of  the  work  to  be  described. 

Sol  j. — This  was  made  by  dissolving  copper  sulphate  in 
water  (amounts  used  as  above),  precipitating  by  means  of 
hydrogen  sulphide,  repeated  washing  by  decantation  for  a 
month,  and  finally  dispersing  by  repeated  treatment  with 
hydrogen  sulphide  accompanied  by  agitation.  This  sol  main- 
tained itself  in  good  condition,  showing  no  settling  after  one 
year. 

Sol  4. — This  was  prepared  in  the  same  way  as  Sol  3,  except 
that  the  copper  sulphate  was  strongly  acidified  with  sul- 
phuric acid  before  the  precipitation  of  the  sulphide.  Upon 
attempting  to  disperse  the  sulphide  thus  formed  after  wash- 
ing as  for  Sol  3,  the  result  was  unsatisfactory,  the  dispersion 
not  forming  at  all  readily.  The  precipitate  was  therefore 
dialyzed  for  two  weeks,  after  which  it  yielded  to  dispersion 
with  the  same  ease  as  that  used  in  preparing  Sol  3. 


Roland  Neal 


Sol  4  (a). — This  was  in  all  respects  a  duplicate  of  Sol  4. 

Sol  5. — This  was  prepared  by  adding  sufficient  ammonia 
to  the  copper  sulphate  solution  to  redissolve  $11  copper  hy- 
droxide, before  precipitation  with  hydrogen  sulphide.  Other- 
wise the  preparation  was  identical  with  Sol  4. 

Effect  of  Electrolytes  on  the  Rate  and  Degree  of  Dispersion 
of  Copper  Sulphide  by  Hydrogen  Sulphide. — While  the  floccula- 
tion  of  colloidal  solutions  by  electrolytes  has  been  extensively^ 
studied,  little  is  known  of  the  influence  of  electrolytes  in  in- 
hibiting dispersion.  Considerable  time  was  devoted  to  an 
attempt  to  get  some  light  on  this  point,  but  it  was  soon  evident 
that  the  experiments  would  demand  an  elaborateness  of 
technique  and  apparatus  that  would  make  an  extensive 
investigation  in  this  direction  impracticable  in  the  time  at 
disposal.  Before  abandoning  this  work,  which  it  is  the  in- 
tention to  take  up  later  in  more  thorough  manner,  some 
results  sufficiently  interesting  to  be  recorded  here  were  ob- 
tained. 

The  method  employed  was  to  place  in  each  of  several  test- 
tubes  the  same  small  amount  of  copper  sulphide  (actually  the 
carbonate,  thoroughly  washed,  was  used).  To  the  first  were 
then  added  5  cc  of  pure  water,  to  each  of  the  others  were  added 
5  cc  of  the  electrolytes  to  be  studied  each  at  its  desired  con- 
centration, this  being,  of  course,  always  smaller  than  that 
necessary  to  produce  flocculation.  Hydrogen  sulphide  was 
then  bubbled  slowly  through  and  the  dispersion  determined 
by  observation  from  time  to  time.  In  all  cases  where  dis- 
persion took  place  it  seemed  to  be  complete  after  twenty-four 
hours.  The  results  were,  in  general,  that  in  like  concentra- 
tions the  inhibitions  to  dispersion  were  in  the  same  order  as  the 
flocculating  powers  of  the  electrolytes,  as  is,  of  course,  to  be 
expected.  One  marked  peculiarity  was  however  found  in  the 
unexpected  action  of  the  alkaline  hydroxides.  These  are 
usually  held  to  stabilize  the  sulphide  sols  up  to  a  certain  point, 
but  in  our  work  they  seemed  to  show  the  same  inhibition  of 
dispersion  as  the  equivalent  solutions  of  alkaline  chlorides. 
Curiously  enough,  however,  they  did  show  a  marked  initial 


Colloidal  Solutions  of  Copper  Sulphide  7 

acceleration  in  the  rate  of  dispersion,  and  probably  a  marked 
initial  increase  in  the  degree. 

If  three  tubes  were  taken,  as  above,  all  containing  a 
small  amount  of  copper  carbonate,  one  containing  also  pure 
water,  the  other  two  containing  N/iooo  KC1  and  N/iooo  KOH, 
respectively,  and  hydrogen  sulphide  bubbled  through  all  three 
simultaneously,  the  one  containing  the  potassium  hydroxide 
would  show  a  far  more  rapid  and  extensive  dispersion  than  the 
others,  while  after  a  few  hours  this  would  have  reflocculated 
to  a  great  extent  and  the  condition  of  the  tube  after  24  hours 
would  be  about  the  same  as  that  of  the  one  containing  potas- 
sium chloride.  In  the  mean  time,  the  dispersion  in  the  tube 
without  electrolyte  will  have  far  outdistanced  the  other  two. 

While  there  seems  to  be,  at  present,  no  satisfactory  ex- 
planation of  this  phenomenon,  it  is  believed  that  there  may  be 
found  in  this  sort  of  experiments  the  nucleus  for  an  interesting 
method  of  studying  the  reverse  reaction  to  flocculation. 

Effect  of  Removal  of  Hydrogen  Sulphide  from  Copper 
Sulphide  Sols. — Some  experiments  were  carried  out  to  de- 
termine to  what  extent  the  stability  of  a  copper  sulphide  sol  is 
dependent  on  the  continued  presence  of  hydrogen  sulphide. 
In  this  work  the  apparatus  described  by  Young  and  Goddard1 
was  used.  In  the  parchment  bag  was  placed  a  one-thousandth 
normal  solution  of  copper  acetate  and  hydrogen  sulphide  was 
bubbled  continuously  through  the  surrounding  dialyzing  water. 
It  slowly  diffused  into  the  acetate  solution  forming  the  sul- 
phide, while  the  acetic  acid  formed  gradually  dialyzed  out- 
ward. While  there  was  at  first  some  flocculated  sulphide 
formed,  this  gradually  dispersed  under  the  influence  of  the 
stirring  and  of  the  hydrogen  sulphide,  until  after  two  days  of 
treatment  at  intervals,  the  sol  showed  no  tendency  to  settle. 
When  this  condition  had  been  reached,  the  hydrogen  sulphide 
water  in  the  outer  compartment  was  replaced  by  pure  water, 
and  this  replacement  was  repeated  every  day  for  ten  days, 
thus  ensuring  a  practically  complete  removal  of  free  hydrogen 


1  Jour.  Phys.  Chem.,  21,  3  (1917). 


8  Roland  Neal 

sulphide  from  the  sol.  There  was  no  sign  of  settlement  at 
the  end  of  the  ten  days,  nor  was  there  after  an  additional 
twelve  days  of  standing.  After  this,  the  apparatus  was 
opened  and  a  sample  removed,  which  was  placed  in  a  tightly 
stoppered  bottle.  There  was  no  odor  of  hydrogen  sulphide. 
After  standing  for  five  months  this  sample  showed  consider- 
able flocculation  and  settlement,  which  in  no  case  happened 
with  sols  kept  saturated  with  hydrogen  sulphide,  even  after  a 
period  of  a  year. 

To  the  sol  remaining  in  the  dialyzing  bag  there  was  added 
a  fresh  amount  of  copper  acetate  solution,  sufficient  to  make 
the  whole  about  three  thousandths  normal.  This  was  sub- 
jected to  the  same  treatment  as  the  first  solution.  After  com- 
plete dispersion  had  taken  place,  and  the  hydrogen  sulphide 
had  been  completely  dialyzed  out,  the  sol  was  bottled  and 
stoppered  and  set  away  for  observation.  After  two  months 
it  was  seemingly  unchanged,  but  after  four  months  it  was 
almost  wholly  flocculated.  It  is  thus  quite  certain  that  copper 
sulphide  sols  depend  for  their  stability  upon  the  presence  of 
hydrogen  sulphide,  which  agrees  with  the  results  of  Young 
and  Goddard1  on  the  other  sulphides.  They  may,  however, 
maintain  themselves  in  the  form  of  the  unstable  dispersion  for 
long  periods  of  time. 

Flocculation  by  Electrolytes. — The  literature  concerning  the 
flocculation  of  sulphide  sols  is  so  extensive  and  so  well  known 
that  it  need  not  be  recited  here.  As  particularly  concerns 
copper  sulphide  sols,  Spring  and  du  Boeck2  carried  out  floccula- 
tion tests  on  this  substance,  the  method  being  to  add  10  drops 
of  the  sol  to  10  cc  of  solutions  of  various  concentrations  of  the 
electrolytes  to  be  investigated.  In  the  stronger  electrolyte 
solutions  distinct  flocculation  or  turbidity  appears,  in  the 
weaker  ones  it  does  not,  a  tube  to  which  10  drops  of  the  sol 
had  been  added  to  10  cc  of  pure  water  being  used  as  a  com- 
parison tube.  The  electrolytes  arranged  themselves  in  the 


1  Loc.  cit. 

2  Bull.  Soc.  roy.  belg.  (2)  47,  165  (1887). 


Colloidal  Solutions  of  Copper  Sulphide  9 

same  general  order  of  flocculating  power  as  was  found  later  by 
Freundlich1  in  his  study  of  arsenic  sulphide  sols,  and  as  had 
been  found  previously  by  Schultze2  for  antimony  sulphide 
sols  even  though  the  experimental  methods  employed  by  the 
different  investigators  varied  considerably. 

Flocculation  experiments  were  carried  out  in  this  work, 
chiefly  to  determine  whether  or  not  the  sols  from  different 
sources  conducted  themselves  materially  differently  from  one 
another.  The  method  used  was  to  prepare  a  solution  of  a 
given  electrolyte  of  a  concentration  double  that  which  was 
desired  in  the  final  solution.  This  was  saturated  with  hy- 
drogen sulphide,  after  which  2  cc  of  it  were  mixed  with  2  cc 
of  the  sol  to  be  investigated.  After  mixing,  hydrogen  sul- 
phide was  again  passed  through  the  mixture  for  a  few  moments, 
after  which  the  tube  was  tightly  stoppered  and  set  away  for 
observation.  The  period  of  standing  which  was  chosen  was 
twenty-four  hours.  If  at  the  end  of  that  time  the  liquid  was 
still  colored  and  turbid,  it  was  considered  to  be  not  completely 
flocculated.  In  all  cases  in  the  following  results,  flocculation 
means  that  the  settlement  was  complete,  the  liquid  water- 
white  and  not  opalescent.  The  period  of  twenty-four  hours 
was  chosen  instead  of  a  shorter  one  (Freundlich  allowed  one- 
half  hour)  because  a  set  of  preliminary  tests  showed  that  the 
results  were  more  consistent  after  the  longer  period.  The 
presence  of  hydrogen  sulphide  in  the  solution  and  an  atmos- 
phere of  hydrogen  sulphide  above  it,  were  found  to  greatly 
increase  the  regularity  of  the  results  obtained.  Potassium, 
calcium  and  aluminum  chlorides  were  investigated  with  regard 
to  their  flocculating  effects.  Sols  i  and  2  (see  ante)  were 
used,  two  sets  of  experiments  being  made  with  Sol  2,  and  Sol 
i  being  investigated  at  full  concentration  and  in  dilutions  of 
1-5,  i -10,  1-15  and  1-20,  the  dilution  being  in  all  cases  made 
with  water  saturated  with  hydrogen  sulphide.  The  results 
are  shown  in  Table  I.  (  +  )  indicates  complete  flocculation 
and  ( — )  indicates  incomplete  flocculation. 


1  Zeit.  phys.  Chem.,  44,  129  (1903). 

2  Jour,  prakt.  Chem.,  (2)  25,  431  (1882). 


IO 


Roland  Neal 


TABLE  I 

Concentrations  of  Electrolytes  Necessary  to  Completely  Flocculate 
Copper  Sulphide  Sols 


Sol 

K 

Cl 

Ca 

Cl, 

Al( 

2i 

(+) 

(—  ) 

(+) 

(-) 

(+) 

(—  ) 

3N 

2N 

8AT 

7N 

3N 

2N 

2 

100 

4N 

IOO 

3N 

IO    OOO 

loN 

IO    OOO 

9N 

IOO    000 

3JV 

IOO    OOO 

2N 

100 

3N 

IOO 

2N 

10    000 

loN 

10    000 

9N 

IOO    OOO 

5^V 

IOO    OOO 

4N 

MT     _, 

100 

IOO 

IO    OOO 

9AT 

IO    OOO 

8N 

IOO    OOO 

IOO    000 

•  I  5 
~p\ii    T   T/~. 

IO    OOO 

9N 

IO    OOO 

SN 

Ull.    I     IO 
Mr      T  r 

10    000 

9N 

10    000 

SN 

•    T      T5 
T^lil      T     or» 

IO    OOO 

gN 

IO    OOO 

SN 

LJii.    I     2O 

10    000 

10    000 

When  compared  with  the  results  of  Spring  and  du  Boeck1 
the  results  show  good  agreement  so  far  as  potassium  chloride 
is  concerned.  The  results  for  calcium  chloride  are  about 
25  percent  lower  than  theirs  for  barium  chloride  (they  give 
none  for  the  calcium  salt)  while  the  results  for  aluminum 
chloride  are  50  percent  or  more  lower  than  theirs  for  aluminum 
sulphate.  Since  the  anion  has  but  little  influence  on  the 
flocculating  power  of  an  electrolyte  for  sulphide  sols,  it  might 
reasonably  be  expected  that  the  results  for  aluminum  chloride 
and  sulphate  should  show  closer  agreement.  The  discrepancy 
is,  however,  probably  to  be  explained  by  the  longer  time  given 
for  reaching  equilibrium  in  this  work,  as  well  as  by  the  rather 
fundamentally  different  manner  in  which  the  experiments  were 
carried  out. 

The  results  of  the  flocculation  experiments  show  that  the 
amount  of  electrolyte  required  does  not  vary  appreciably  with 


1  Loc.  cit. 


Colloidal  Solutions  of  Copper  Sulphide  n 

the  source  and  manner  of  preparation  of  the  sol,  and  that  the 
amount  of  electrolyte  required  is  independent  of  the  dilution 
of  the  sol  within  wide  limits.  This  latter  was  found  to  be  true 
within  rather  close  limits  by  Freundlich1  for  arsenic  sulphide 
sols. 

Flocculation  tests  were  also  made  upon  sols  from  which 
the  hydrogen  sulphide  had  been  removed  by  dialysis.  The 
results  were  the  same  as  those  obtained  when  the  sols  were 
saturated  with  hydrogen  sulphide,  for  which  reason  it  is  not 
considered  necessary  to  give  numerical  data  concerning  them. 

From  the  results  obtained  in  this  work  the  relative 
flocculating  powers  of  potassium,  calcium,  and  aluminum 
for  copper  sulphide  sols  are: 

1:39:875 

Rate  of  Migration  in  the  Electric  Field. — The  rate  of 
migration  of  the  copper  sulphide  in  the  sols  under  the  in- 
fluence of  the  electric  field  was  studied  quite  extensively  in 
order  to  determine  the  following  factors : 

(a)  Reproducibility  of  results  in  successive  experiments 
using  the  same  sol  under  the  same  conditions. 

(b)  The  influence  of  the  origin  of  the  sol  on  its  rate  of 
migration. 

(c)  The  influence  of  dilution. 

(d)  The  influence  of  electrolytes. 

(e)  The  influence  of  hydrogen  sulphide. 

(/)  The  combined  influence  of  hydrogen  sulphide  and 
electrolytes. 

(g)    The  influence  of  oxygen. 

The  apparatus  used  in  these  experiments  is  shown  in 
Fig.  i  and  is  practically  self-explanatory.  Small  platinum 
wires  were  used  as  electrodes  and  these  were  allowed  to  dip 
about  2-3  mm  below  the  surface.  Readings  were  not  begun 
until  a  well-defined  flat  migration  surface  had  established 
itself.  In  order  to  avoid  the  effect  of  light,  the  measurements 
were  made  in  a  darkened  chamber.  The  voltage  employed 


1  Loc  cit. 


12 


Roland  Neal 


was  as  near  as  possible  to  100.  With  the  equipment  at  hand, 
it  was  not  always  possible  to  maintain  this  with  exactness, 
and  many  of  the  results  are  calculated  to  this  voltage  on  the 
assumption  that  the  migration  rate  is  proportional  to  the 
potential  fall.  It  has  been  the  experience  in  this  laboratory 
that  attempts  to  have  the  electrodes  bathed  in  pure  water 
containing  none  of  the  colloidal  material 
lead  to  more  fluctuating  results  than  are  ob- 
tained when  such  device  is  not  used,  espe- 
cially when  the  sols  contain  electrolytes  or 
other  easily  diffusible  materials.  This  is 
probably  due  to  the  fact  that  the  conductor 
is  no  longer  homogeneous  as  well  as  to  the 
fact  that  electrolytes  diffuse  out  into  the 
pure  water  and  materially  alter  the  con- 
ditions at  the  surface  of  migration.  While 
dipping  the  electrodes  directly  in  the  sol 
means  increased  electrolysis,  it  is  neverthe- 
less true  that,  judging  from  reproducibility 
and  consistency  of  results,  it  is  the  lesser 
evil  which  is  encountered  in  this  way.  All 
the  results  given  below  were  obtained  with 
electrodes  dipping  directly  in  the  sol. 

(a)  The  Reproducibility  of  Results.— 
For  the  purpose  of  determining  the  repro- 
ducibility of  measurements  on  one  and  the 
same  sol  in  different  determinations,  a  por- 
tion of  Sol  4  was  diluted  with  21/z  vols. 
of  hydrogen  sulphide  water,  and  the  whole 
thoroughly  saturated  with  hydrogen  sulphide.  Five  inde- 
pendent determinations  of  the  migration  rate  were  made 
on  the  same  day  and  under  identical  conditions.  Others  were 
made  at  various  intervals  of  time,  often  months  apart.  The 
results  showed  that  the  data  obtained  on  the  same  day  were 
in  very  close  agreement,  but  that  the  rate  of  migration  of 
the  same  sol  might  change  quite  materially  with  time.  In  the 
following,  therefore,  all  measurements  intended  for  com- 


Fig.  i 


Colloidal  Solutions  of  Copper  Sulphide  13 

parison  were  made  on  the  same  day,  as  far  as  possible.  In 
considering  the  whole  set  of  results  to  be  given,  it  is  necessary 
to  bear  in  mind  that  results  obtained  with  the  same  sol  at 
widely  separated  times,  are  not  in  general  strictly  comparable. 
(6)  Influence  of  the  Origin  of  the  Sol  on  the  Migration  Rate. 
— A  considerable  number  of  comparisons  of  migration  rates 
of  sols  of  different  origin  were  made,  with  the  result  that  in 
general  the  rates  found  were  in  no  case  in  agreement,  the 
magnitude  of  the  disagreement  often  being  very  great.  For 
illustration  there  are  shown  in  Chart  I  the  values  obtained  for 


Sols  3,  4  and  5.  It  will  be  recalled  that  Sol  4  was  made  by 
dispersing  copper  sulphide  precipitated  from  acid  copper 
sulphate  solution,  while  Sol  5  was  made  by  dispersing  copper 
sulphide  precipitated  from  an  ammonia  solution  of  copper 
sulphate.  Sol  3  was  similarly  prepared  from  neutral  copper 
sulphate  solution.  The  ordinates  represent  distances  traveled 
in  centimeters  and  the  abscissae  represent  times.  As  will  be 
seen,  Sol  5  migrates  about  three  times  as  rapidly  as  Sol  4. 

This  phase  of  the  work  was  not  carried  further,  the  present 
purpose  being  rather  to  determine  what  factors  could  cause 
variations  in  the  conduct  of  the  sols,  than  to  make  a  detailed 
study  of  them.  It  is  the  intention  to  extend  this  work  in  the 
near  future. 

(c)  The  Influence  of  Dilution. — A  number  of  measure- 
ments of  the  rate  of  migration  of  a  sol  in  its  full  concentration 


14  Roland  Neal 

and  when  diluted  were  made.  In  order  to  alter  the  condition 
of  the  sol  as  little  as  possible  the  dilution  was  always  carried 
out  with  water  saturated  with  hydrogen  sulphide,  and  the  gas 
bubbled  through  the  diluted  sol  immediately  after  dilution, 
to  ensure  complete  saturation. 

It  was  found  that  the  effect  of  dilution  was  invariably 
to  increase  the  rate  of  migration.  The  results  of  two  such 
sets  of  measurements  are  shown  in  Chart  II.  Here,  as 


throughout  the  rest  of  this  paper,  it  is  not  considered  necessary 
to  print  the  figures  for  the  actual  measurements  obtained,  for 
the  actual  values  may  be  read  off  from  the  graphs  with  suffi- 
cient accuracy.  The  data  in  this  case  are  for  Sol  4  in  full 
strength  and  diluted  i  to  i.  The  migration  rate  of  the 
diluted  sol  is  nearly  double  that  of  the  undiluted. 

(d)  The  Influence  of  Electrolytes. — The  influence  of  elec- 
trolytes upon  the  migration  rate  of  copper  sulphide  sols  was 
subjected  to  a  rather  extensive  study.  Potassium  and  calcium 
chlorides  were  investigated  at  less  than  their  flocculating 
concentrations,  and  a  considerable  number  of  concentrations 
of  each  was  investigated.  In  this  way  there  were  obtained 
results  from  which  the  relative  effects  of  the  two  electrolytes 
in  equivalent  concentrations  could  be  obtained.  All  of  the 
different  sols  were  investigated  and  the  influencing  factor 
seemed  to  be  fairly  independent  of  the  original  migration 
rate  of  the  sol.  That  is,  whatever  the  initial  value  for  the 
migration  of  the  sol,  this  seemed  to  be  multiplied  to  about  the 
same  extent  by  the  same  addition  of  electrolyte.  Variations 
of  the  rates  for  the  different  sols  with  time  and  other  causes 


Colloidal  Solutions  of  Copper  Sulphide 


have  made  it  impossible  up  to  the  present  to  obtain  a  wholly 
consistent  set  of  results.  For  the  most  part  the  results  on  a 
given  chart  were  obtained  in  a  very  short  period  of  time, 
after  a  laborious  series  of  preliminary  measurements  had  made 
evident  the  danger  of  comparing  results  obtained  at  very 
considerable  intervals  of  time. 

A  sufficient  number  of  data  to  indicate  clearly  the  main 
results  obtained  have  been  compiled  and  are  shown  in  the 
following  charts:  Chart  III  shows  the  effect  of  successive 
additions  of  potassium  chloride  to  Sol  4.  The  effect  is  to 
increase  the  migration  rate.  Chart  IV  shows  the  effect  of 


CHART 
ScL4H2S  dialled  out 


MINVTL5 


10 


30 


potassium  chloride  on  the  same  sol  after  it  had  been  previously 
dialyzed  for  a  long  period  so  as  to  remove  all  hydrogen  sul- 
phide. It  will  be  noticed  that  the  migration  rate  of  the  sol 
itself  as  well  as  mixtures  of  it  with  potassium  chloride  solu- 


i6 


Roland  Neal 


tions  are  all  higher  than  the  rates  for  corresponding  solutions 
saturated  with  hydrogen  sulphide,  a  matter  to  which  reference 
will  be  made  later.  The  effect  of  the  potassium  chloride  is,  how- 
ever, the  same  in  both  cases,  namely,  to  increase  the  migration 
rate.  Chart  V  shows  the  effect  of  successive  additions  of 
potassium  chloride  to  Sol  5,  a  sol  of  a  high  migration  rate. 
The  chart  shows  that  the  increase  in  migration  rate  is  ap- 
proximately proportional  to  the  amount  of  potassium  chloride 
added.  Comparing  Chart  V  with  Chart  III  it  is  seen  that 


the  increase  of  migration  rate  produced  in  the  two  cases  by 
the  same  addition  of  potassium  chloride  is  quite  closely  pro- 
portional to  the  original  migration  rate  of  the  pure  sol.  Thus 
at  the  end  of  fifteen  minutes,  the  distance  migrated  by  Sol  4 


Colloidal  Solutions  of  Copper  Sulphide 

is  i  cm,  that  by  Sol  5  is  3  cm  and  the  ratio  l/$  =  °-333  +  - 

the  distances  migrated  at  the  end  of  the  same  period  by  the 

same  sols  to  which  potassium  chloride  has  been  added  in 

I  I OOO 

normality,  we  have,  respectively,  1.5  cm  and  4.5  cm,  and  the 
ratio  =  0.333  +  .  This  is  a  fortunately  chosen  case,  but  in 
general  the  rule  seems  to  hold  fairly  closely.  The  influence 
of  the  potassium  chloride  on  the  sol  from  which  hydrogen 
sulphide  had  been  removed  is  seen  to  be  much  smaller. 

The  effect  of  calcium  chloride  is  shown  in  Chart  VI. 
It  is  entirely  similar  to  potassium  chloride  in  its  conduct. 
One  thousandth  normal  is  about  the  concentration  of  calcium 
chloride  that  will  flocculate  the  sol  in  twenty-four  hours; 
in  fact,  it  is  a  trifle  above  it.  It  was  nevertheless  possible 
to  carry  out  the  migration  measurements  at  this  concentra- 
tion, although  flocculation  occasionally  occurred  during  the 
experiment. 

Chart  VII  shows  the  relative  influence  on  Sol  5  of  potas- 


10        15       20       25       30 


sium  and  calcium  chlorides  at  one-thousandth  normal  con- 
centrations, the  graph  for  the  pure  sol  being  added  for  com- 
parison. As  will  be  seen,  the  effect  of  the  potassium  chloride 
somewhat  exceeds  that  of  the  calcium  chloride,  although  but 
little.  There  is  thus  no  apparent  relation  between  the 
flocculating  power  and  the  migration  influence.  One  seems, 
however,  reasonably  justified  in  surmising  that  the  migration 
influence  stands  in  a  fairly  direct  relation  to  some  function 


i8 


Roland  Neal 


of  the  concentrations  of  the  electrolyte  added,  very  possibly 
the  conductivity.  In  this  event  the  acceleration  of  the 
migration  rate  would  be  represented  by  S.K.X  where  K  is  the 
original  rate  of  the  uninfluenced  sol  and  X  the  conductivity 
induced  by  the  added  electrolyte  while  S  is  a  factor  which 
varies  with  the  condition  of  the  particular  sol,  and  is  at  present 
indeterminate.  Before  this  principle  can  be  established  with 
any  degree  of  definiteness,  a  far  more  exhaustive  series  of 
experiments  is  necessary.  These  are  planned  for  the  near 
future.  As  a  matter  of  fact  results  of  similar  experiments  per- 
formed some  years  ago  by  R.  C.  Pollock  and  F.  S.  Pratt  on 
arsenious  sulphide  sols,  the  results  of  which  are  in  preparation 
for  publication,  show  a  quite  different  relationship. 

Effect  of  Hydrogen  Sulphide. — -It  has  been  pointed  out 
above  that  the  effect  of  hydrogen  sulphide  is  to  reduce  the 
rate  of  migration.  Repeated  measurements  were  made  of  this 
effect  which  was  always  found  to  occur,  whether  or  not  elec- 
trolyte were  present.  The  results  of  some  measurements  of 
the  effect  are  given  in  Chart  VIII.  The  reduction  of  the 


4  — 


I—Sat.  w.H^S    no  Electrolyte 

2-"     "      "     n&j 
3-No  M2S  no  EUctrolijIc, 


MINWTtS 


'0 


10 


15 


?0 


25 


30 


migration  rate  by  hydrogen  sulphide  is  found  to  be  very  large. 
The  action  in  this  case  was  found  to  be  reversible,  the  migra- 
tion rate  increasing  when  hydrogen  sulphide  was  dialyzed 
out  and  diminishing  when  it  was  resaturated  with  hydrogen 
sulphide. 


Colloidal  Solutions  of  Copper  Sulphide  19 

Here  again  is  found  evidence  of  the  rule  that  the  in- 
fluence of  the  electrolyte  on  the  migration  rate  is  closely 
proportional  to  the  value  of  this  quantity  for  the  sol  without 
electrolyte.  The  distances  on  Curve  2  are  to  those  on  Curve 
i,  taken  at  the  same  time  as  i  :  1.35.  Those  on  Curve  4 
are  to  those  on  Curve  3  as  i  :  1.34.  The  different  value  for 
this  ratio  from  that  obtained  from  the  same  sol  saturated 
with  hydrogen  sulphide  illustrates  the  character  of  S  in  the 
formula  suggested  above. 

Influence  of  Oxygen.- — Curing  the  course  of  the  investiga- 
tion it  was  considered  wise  to  determine  whether  or  not  free 
oxygen  played  any  part  in  influencing  the  migration  rate. 
The  results  were  negative.  Air  or  oxygen  bubbled  through  a 
sol  freed  from  hydrogen  sulphide  by  dialysis  showed  not  the 
least  effect.  If  a  sol  saturated  with  hydrogen  sulphide  were 
used  there  occurred  always  an  increase  of  migration  rate 
which  was  restored  to  its  original  value  by  resaturation  with 
hydrogen  sulphide.  If  air  or  oxygen  were  passed  through 
for  a  long  time  the  migration  rate  took  on  a  value  very  close 
to  that  of  the  dialyzed  sol,  from  which  it  was  concluded  that 
the  influence  of  the  air  or  oxygen  was  merely  that  due  to 
displacement  of  hydrogen  sulphide. 

Summary 

Copper  sulphide  sols  were  prepared  in  a  number  of  different 
ways  and  their  conduct  with  respect  to  flocculation  by  elec- 
trolytes as  well  as  their  rates  of  migration  in  the  electric  field 
were  studied.  The  following  conclusions  are  indicated  by  the 
data  obtained : 

(1)  The  concentration  of  electrolyte  necessary  to  floccu- 
late in  twenty-four  hours  is  independent  of  the  origin  of  the 
sol. 

(2)  It  is  within  wide  limits  independent  of  the  dilution  of 
the  sol. 

(3)  It  is  independent  of  the  presence  or  absence  of  free 
hydrogen  sulphide  in  the  sol. 

(4)  The  relative  flocculating  powers  of  the  chlorides  of 
potassium,  calcium  and  aluminum  are  as  i  :  39  :  875. 


2O  Roland  Ncal 

(5)  Evidence   is   found   that   sols   which   have   been   de- 
prived of  hydrogen  sulphide  are  unstable,  and  will  ultimately 
flocculate  spontaneously.     The  time  required  is,  however,  very 
long,    five   or   more   months.     Electrolyte   additions  show  no 
difference  in  the  conduct  of  sols  saturated  and  those  unsaturated 
with  hydrogen  sulphide  (see  3). 

(6)  The  rate  of  migration  of  these  sols  in  the  electric 
field  is  largely  influenced  by  the  origin  of  the  sol. 

(7)  It  is  accelerated  by  the  addition  of  electrolytes,  but 
in  a  way  which  seems  to  bear  no  relation  to  the  flocculating 
value  of  the  electrolyte  used. 

(8)  The  acceleration  of  the  migration  rate  depends  upon 
the  migration  rate  of  the  original  sol,  and  seems  fairly  well 
represented  by  an  expression  of  the  form  S.K.X,  where  K  is 
the  migration  rate  of  the  original  sol  and  X  a  function  of   the 
concentration  (perhaps  of  the  conductivity)  of  the  electrolyte 
added  and  S  is  a  factor  depending  on  some  specific  property 
of  the  particular  sol  used. 

(9)  Other  conditions  being  kept  constant,  the  migration 
rate  is  increased  by  dilution. 

(10)  The  migration  rate  is  greatly  reduced  by  hydrogen 
sulphide,  and  increased  by  its  removal,  the  effect  being  readily 
reversible. 

(n)  Bubbling  air  or  oxygen  through  a  sol  containing 
hydrogen  sulphide  affects  the  migration  rate  merely  by  dis- 
placing hydrogen  sulphide.  Resaturation  with  hydrogen 
sulphide  restores  the  rate  to  its  original  value. 

A  considerable  portion  of  the  experimental  work  involved 
in  this  paper  was  carried  out  at  the  laboratory  of  the  College 
of  the  Pacific,  College  Park,  Cal.,  the  remainder  at  Stanford 
University. 

The  above  work  was  carried  out  at  the  suggestion, 
and  under  the  direction  of  Professor  S.  W.  Young,  Depart- 
ment of  Physical  Chemistry,  Stanford  University. 

Laboratory  of  Physical  Chemistry 
Stanford  University,  California 


Syracuse,  N.  Y 


3C5231 


UNIVERSITY  OF  CAUFORNIA  LIBRARY 


